During the manufacturing of semiconductor-based devices, it is common to etch features, such as a trench, into a substrate. One exemplary film stack employed when etching a trench into a silicon substrate includes a mask composed of a thick silicon dioxide layer on top of a second thin layer, e.g., a silicon nitride layer, which is disposed over the silicon substrate. The silicon nitride layer is typically present as either an etch stop for subsequent CMP applications or as a stress relief for the underlying silicon layer. In addition, a photoresist layer may or may not be disposed on top of the silicon dioxide layer: Dry etch plasma reactors utilizing inductive coupled plasma technology (ICP) designed for silicon etch applications are limited in performing the silicon trench etch. This limitation is due to the thin layer, i.e., the silicon nitride layer, being laterally etched during the etch process. The etching of the thin layer may proceed laterally into the exposed portion of the silicon nitride layer around the trench opening. Though the lateral etching is small relative to the depth of the feature etched, e.g., the trench depth may be 20 microns deep, the lateral etching is significant enough so as to possibly cause voiding during subsequent gap filling of the trench. This voiding can eventually lead to device failure due to the stresses experienced in the vicinity of the void.
In view of the foregoing, there is a need for a method and apparatus that provides for the avoidance of the lateral etching of the thin layer disposed over the substrate into which a feature is being etched.